There is now increasing evidence that pathological pain states are at least in part driven by changes in the brain itself. Descending modulatory pathways are known to mediate top- down regulation of nociceptive processing, transmitting cortical and limbic influences to the dorsal horn. Ascending pain transmission pathways are also intimately intertwined with these modulatory systems, forming positive and negative feedback loops. The output node of the best-characterized pain-modulating system is the rostral ventromedial medulla (RVM). Building on my laboratory's experience defining the outputs of RVM neurons, the studies in the present application fill an important gap, identifying a pathway through which noxious input reaches the RVM. My laboratory has demonstrated that pain-inhibiting and pain-facilitating influences from the RVM are mediated by two classes of neurons, ON-cells, which exert a net facilitating influence on nociception, and OFF-cells, which have a net inhibitory action. The overarching goal of this proposal is to begin to define nociceptive inputs to the RVM. Our preliminary data suggest that one important relay is the parabrachial complex. This region is the main target of nociceptive pathways arising from the superficial dorsal horn. We propose to test the role of the parabrachial complex in regulating the activity of RVM pain- modulating neurons, and to determine how noxious inputs are altered in chronic pain states. These studies will use in vivo single-cell recording from identified RVM ON- and OFF- cells, optogenetics, and pharmacological manipulations to test the hypothesis that the parabrachial complex is a direct relay for noxious input to RVM in acute thermal and mechanical nociception (Aim 1). The same approaches will then be used to determine how these nociceptive inputs to the RVM are modified in a model of chronic inflammatory pain (Aim 2). Complementing this in vivo work, parallel studies under Aim 3 will use in vitro electrophysiology in an adult RVM slice with optogenetic manipulation of defined terminals in the RVM to define the membrane mechanisms of parabrachial input to the RVM and determine how that is altered in chronic inflammation. By defining pathways through which noxious information reaches pain-modulating neurons at the membrane, individual neuron, and circuit level, we can begin to define how pain-modulating circuits are recruited in acute and chronic pain. This information is critical if we are ever to develop treatments addressing chronic pain as maladaptive brain disease.